The present invention relates to the field of radio telecommunications, including cellular telecommunications, and more specifically, to maximizing the signal-to-noise ratio in multicarrier transmitters by controlling the power level of each of the carriers entering a digital-to-analog converter and by controlling overall power gain of the analog portion of the transmitter.
In conventional cellular systems, a base station is allocated a predetermined number of frequency channels for communication with mobile stations. In the base station a separate transmitter is employed for each frequency channel. However, the use of separate transmitters for each frequency channel results in a duplication of parts and an increase in cost due to the additional hardware required. Thereafter, it was realized that the hardware cost per channel could be reduced by using multicarrier transmitters in place of the plurality of single carrier transmitters to transmit a plurality of frequency channels. Since multicarrier transmitters transmit over a broad range of frequencies, they are also sometimes referred to in the art as wideband transmitters. However, for ease of discussion, the transmitters will be referred to herein as multicarrier transmitters.
FIG. 1 illustrates a conventional multicarrier transmitter 100 which might be used to transmit multiple frequency channels from a base station in a radiocommunication system. The conventional multicarrier transmitter 100 operates as follows. A number N of baseband frequency data signals BB1 . . . BBN are modulated by modulators Mod1 . . . ModN, respectively, where the bits associated with each data signal are symbol encoded for transmission, i.e., the modulator generates the corresponding baseband waveform. Each of the modulated data signals is forwarded to a corresponding digital power control module DPC1 . . . DPCN, where each DPC adjusts the signal power level of the corresponding modulated data signal based on the commands provided by the Radio Control Unit 150. More specifically, the power level of each modulated data signal is adjusted such that the absolute power level of each carrier Pk,out at the transmitter is equal to the amount of power required for the carrier to reach a particular mobile station which is to receive the carrier, where k varies from 1 to N and identifies the corresponding baseband frequency data signals BB1 . . . BBN.
The modulated data signals are then forwarded from the digital power control modules DPC1 . . . DPC1 N to multipliers Mult1 . . . MultN, respectively, where each modulated data signal is upconverted to a corresponding carrier frequency. The upconverted signals are then summed by adder 110. The compound signal produced by adder 110 is then forwarded to the digital-to-analog converter (DAC) 120. The resulting compound analog signal is then passed from DAC 120 through an analog transmitter chain which includes analog amplifier 160, upconverter (not shown), and filters (not shown). Analog amplifier 160 then amplifies the compound signal by a fixed gain Gi. For ease of discussion Gi has been described as the gain of analog amplifier 160, however, one skilled in the art will recognize that Gi represents the total gain of the analog section of the transmitter, including losses due to filters and upconverters. A more detailed discussion of multicarrier transmitters can be found in xe2x80x9cBase-Station Technology Takes Software-Definable Approachxe2x80x9d by Richard M. Lober, Wireless System Designs, February 1998, which is herein incorporated by reference.
Multicarrier transmitters are designed to handle a maximum number of simultaneous carriers N. In designing a multicarrier transmitter, care must be taken to ensure that the instantaneous in-phase sum, Psum, of the N carriers does not exceed the full scale range of the DAC, i.e., the value associated with the greatest digital code that can be converted into an analog value. Psum can be calculated using equation (1) below, where CN represents the power of a specific user, N, in a specific time slot on a specific carrier frequency. Normally, CN is equal to the peak power within the specific time slot.
({square root over (C1)}+{square root over (C2)}+ . . . +{square root over (CN)})2=PSUMxe2x80x83xe2x80x83(1)
If the instantaneous sum of the N carriers exceeds the full scale range of the DAC, the DAC will clip the analog signal, i.e., prevent the analog signal from exceeding the amplitude corresponding to the full scale range of the DAC, which will affect the quality of the transmitted signal. However, one skilled in the art will recognize that in practical applications, a system might tolerate a power level which exceeds the DAC""s full scale range by a small amount for short periods of time without suffering a decrease in system performance.
In a multicarrier transmitter with N carriers, the abovementioned xe2x80x9cclippingxe2x80x9d of the analog signal can be avoided by setting the full scale range of the DAC to 20*log(N) dB above the maximum allowed peak power level of any individual carrier 1 . . . N, since the full scale range set 20*log(N) dB above the maximum power level of any individual carrier represents the greatest power level attainable by the sum of the N carriers.
In FIG. 2, the maximum power level associated with each of the N carriers, during a given time slot in a TDMA based system, is Cmax. Accordingly, the full scale range of the DAC is set to 20*log(N) dB above Cmax. When the output noise of the DAC is dominated by quantization noise, the DAC noise floor level relative to the DAC full scale range, is constant for a given DAC resolution and sampling frequency. If all carriers are operating at maximum power, the signal-to-noise ratio for each individual carrier is at its greatest level.
Unlike FIG. 2, FIG. 3 illustrates a time slot where the N carriers have different power levels and not all of the N carriers are active. Assuming the DAC full scale range in FIG. 3 is the same as the DAC full scale range in FIG. 2, the noise floor levels of FIGS. 2 and 3 will also be the same. Since the power levels of the N carriers in FIG. 3 are less than the power levels of the N carriers in FIG. 2, while the noise floor level in FIGS. 2 and 3 is the same, the signal-to-noise ratio for each of the individual carriers in FIG. 3 will be lower than for the corresponding individual carriers in FIG. 2.
Accordingly, it would be desirable to maximize the signal-to-noise ratio for each carrier and time slot. Further, it would be desirable to maximize the signal-to-noise ratio for each individual carrier in a multicarrier transmitter when less than the maximum number of carriers are used and/or when not all carriers are operating at the maximum power level. In addition, it would be desirable to maximize the signal-to-noise ratio of each individual carrier in the multicarrier transmitter without the instantaneous sum of all the carriers exceeding the DAC""s full scale range.
The power level of individual carriers is adjusted in the digital domain, by a command from a radio control unit, such that Psum is equal to the full scale range of the DAC, or such that Psum exceeds the full scale range of the DAC, though not enough to cause intolerable distortion to the signal. The radio control unit also commands an analog power control module to adjust the power level of all carriers simultaneously such that the transmitted actual absolute output power level of each carrier is set to the correct level for a corresponding mobile station to receive an acceptable signal.
It is an object of the present invention to improve the signal-to-noise ratio of a multicarrier signal which passes through a DAC.
It is also an object of the present invention to maximize the utilization of a DAC in a multicarrier transmitter system, thereby reducing the implementation cost of the transmitter chain in a radio base station.
It is a further object of the present invention to maximize the signal-to-noise ratio of each individual carrier in a multicarrier transmitter, thereby improving end user quality of service for all users.